1. Field of the Invention
The present invention relates to the field of numerically control machines and in particular to a numerically controlled tapping machine having an independent drive motor for the tap.
2. Description of the Prior Art
Numerically controlled machines typically use a motor driven spindle which is operated by a computer. It is known in the art to sense a spindle function, such as thrust of the spindle, and perform some control function in response to this information. See Alexander et al., U.S. Pat. No. 4,854,786 and Eckman, U.S. Pat. No. 4,688,970.
However, in a tapping machine it is necessary for the tapping tool to rotate in a first sense as its taps downwardly into the bore to be threaded, stop when the threading is compete and then reverse it sense rotation as the tap is removed from the threaded bore. In the prior art, the advance of the spindle into the workpiece has been coordinated with control of the rotation of the spindle according to predetermined calculations or assumptions made with respect to the permissible rate of advance of the tapping tool within the workpiece. Clearly, a vertical advance or retreat of the spindle at a rate which is either excessively fast or slow compared to the rate which the tapping tool can cut threads into or unscrew from the threaded bore, at the very least places stress upon the tapping tool and machine threads, causes excessive and premature wear of the tapping tool, and in the worst case either breaks the tool or strips the threads.
Therefore, what is needed is some type of method and apparatus whereby the operation of the tool on a spindle can be independently controlled and in particular some type of method and mechanism for appropriately driving a tapping tool both when cutting threads as well as unscrewing from the threads just cut.